1. Field of the Invention
The present invention relates to an image pickup apparatus and an image pickup method for acquiring bright-time image data and dark-time image data to correct dark current.
2. Description of the Related Art
When an S/N ratio decreases, image noise caused by dark current is conspicuous, for example, in the case of acquiring an image by performing long time period exposure of an object with a low luminance. For removal of such dark current noise, various proposals have been provided conventionally.
For example, in Japanese Patent Application Laid-Open Publication No. 2000-209506, a technique is described in which, in view of a fact that dark current increases or decreases according to a temperature of an image pickup device or an exposure time period, image quality deterioration caused by the dark current is reduced by performing a charge storage operation for a same exposure time period as a bright-time image to acquire a dark-time image due to a dark-current component in a state that the whole image pickup device is shielded from light, immediately after the bright-time image is picked up, and subtracting the dark-time image from the bright-time image.
In Japanese Patent Application Laid-Open Publication No. 2004-229032, a technique as shown below is described which is for reducing troublesomeness of performing photographing of a dark-time image accompanying photographing of a bright-time image. That is, dark-exposure photographing is performed multiple times with different combinations of a shutter speed and camera sensitivity, and obtained multiple pieces of dark image data are stored in a memory before shipment of a digital camera. Then, at the time of performing actual photographing, noise correction data corresponding to a shutter speed and camera sensitivity at the time of obtaining actually photographed image data is calculated on the basis of the dark image data read from the memory, and noise is removed by subtracting the calculated noise correction data from the actually photographed image data.
As a technique making it possible to confirm a state of an image being exposed, at the time of performing long-time-period exposure, a so-called live bulb photographing technique is proposed and productized. The live bulb photographing is a technique making it possible to observe a state of an image being exposed by time-dividing a long-time-period exposed image into multiple exposed images, and reading and displaying the exposed images.
A process for removing dark current noise in the live bulb photographing will be described with reference to FIGS. 24 and 25. FIG. 24 is a timing chart showing an operation at the time of performing live-bulb photographing of a bright-time image, and FIG. 25 is a timing chart showing an operation at the time of performing live-bulb photographing of a dark-time image. Note that FIGS. 24 and 25 illustrate a case of dividing a whole exposure time period Texp into three divided exposure time periods Texp1, Texp2 and Texp3.
As shown in FIG. 24, the live bulb photographing of a bright-time image is started by opening a mechanical shutter (hereinafter “mechanical” will be appropriately abbreviated as “mecha”) and applying a reset pulse RST to pixels to perform reset.
Then, when the first divided exposure time period Texp1 has elapsed after start of exposure, first pixel reading is performed by starting application of a read pulse RD in synchronization with a vertical synchronizing signal VD, and a first piece of time-division bright-time image data A1 is acquired as image pickup data. The acquired piece of time-division bright-time image data A1 is displayed on a display section of an image pickup apparatus after necessary image processing is performed.
Next, when the second divided exposure time period Texp2 has elapsed after the first pixel reading, second pixel reading is performed by starting application of a read pulse RD in synchronization with a vertical synchronizing signal VD, and a second piece of time-division bright-time image data A2 is acquired as image pickup data. The acquired piece of time-division bright-time image data A2 is added to the first piece of time-division bright-time image data A1 and displayed on the display section of the image pickup apparatus after necessary image processing is performed. Thereby, an image equal to an image obtained by performing exposure for an exposure time period (Texp1+Texp2) from an exposure start time point is displayed on the display section.
Furthermore, when the third divided exposure time period Texp3 has elapsed after the second pixel reading, third pixel reading is performed by starting application of a read pulse RD in synchronization with a vertical synchronizing signal VD, and a third piece of time-division bright-time image data A3 is acquired as image pickup data. Note that the mecha shutter is closed when reading of all pixels ends. The acquired piece of time-division bright-time image data A3 is added to the first and second pieces of time-division bright-time image data A1 and A2 (that is, a bright-time image data A=A1+A2+A3 is calculated) and displayed on the display section of the image pickup apparatus after necessary image processing is performed. Thereby, an image equal to an image obtained by performing exposure for the whole exposure time period Texp from the exposure start time point is displayed on the display section.
Immediately after such bright-time image pickup is performed, dark-time image pickup is performed without delay so that dark current having a correlation as high as possible with dark current at the time point of having picked up the bright-time image can be acquired.
The dark-time image acquisition is performed as shown in FIG. 25, and a process up to acquisition of image pickup data is similar to that of the live bulb photographing of the bright-time image except that image pickup is performed in a state of the mecha shatter being closed. However, time-division dark-time image data acquired by time-division exposure is not displayed on the display section unlike the pieces of time-division bright-time image data.
All pieces of time-division dark-time image data B1, B2 and B3 are acquired in this way, these are added together to calculate dark-time image data B=B1+B2+B3 equal to dark-time image data obtained by performing exposure for the whole exposure time period Texp from the exposure start time point. Then, by subtracting the dark-time image data B=B1+B2+B3 from the bright-time image data A=A1+A2+A3, image data for which a dark-current component has been reduced. After that, gain-up and other image processes are performed, and the image data is stored.
By such a process, it becomes possible to confirm a state of an image being formed during bulb photographing by a live view, and to acquire an image for which fixed pattern noise (FPN) has been reduced.
The live bulb photographing as described above is on the assumption of being performed with the image pickup apparatus being fixed by a tripod or the like because a long exposure time period is required.
In the case of performing the live bulb photographing as described above in a hand-held state without using a tripod or the like, image stabilization is thought to be indispensable.
The image stabilization is classified as mecha image stabilization in which a lens or an image pickup device is driven in an image stabilization direction and electronic image stabilization in which a part of an image is cut out so that a certain object part is always included, according to movement of an object image relative to an image pickup surface of the image pickup device.
Since the mecha image stabilization is such an image stabilization that is performed so that a position of an optical object image relative to an image pickup device is kept constant, it is possible to remove FPN by subtracting a dark-time image from a bright-time image as described above.